1. Field of the Invention
This invention relates to a process for differential pressure casting, and more particularly, to one which can produce castings of improved quality.
2. Description of the Prior Art
When a casting is produced from an easily oxidizable material, e.g., aluminum or its alloy, by e.g., a low pressure casting process, it is necessary to prevent the oxidation of molten aluminum or its alloy in a stoke, or to remove aluminum oxide therefrom. It has been usual for the low pressure casting of aluminum or its alloy to provide a gate in the lower portion of mold with a wire net for removing aluminum oxide. Although the wire net may retain coarse oxide particles, it allows the passage of fine oxide particles into a mold cavity, where they contact air and undergo further oxidation. The net needs to be changed to a new one after each casting operation. Moreover, the net adheres to a scrap of aluminum or its alloy cut from a cast product in the gate and thereby disables the effective use of the scrap.
There has been proposed a process for differential pressure casting in which an inert gas is supplied into the mold cavity prior to the pouring of molten metal thereinto for preventing its oxidation, as disclosed in, for example, Japanese Patent Application laid open under No. Sho 48-49625, or Hei 3-71964. This process makes it possible to prevent the oxidation of molten metal without relying upon any wire net provided in the gate. Japanese Patent Application laid open under No. Hei 1-104457 discloses a process for differential pressure casting which employs a stoke comprising an inner cylindrical wall having an upper end connected to the gate in the mold and a lower end kept above the surface of molten metal in a holding furnace, and an outer cylindrical wall having an upper end connected to the inner wall and a lower end immersed in the molten metal. Thus, the stoke has a cylindrical space having a closed upper end and an open lower end defined between the inner and outer walls. Further, the stoke has a hole communicating the cylindrical space with the outside of the stoke. Using this stoke an inert gas, that is heavier than air, is supplied through the hole to remove air from the stoke, gate and mold cavity, then the molten metal is introduced from the holding furnace into the mold cavity by a difference of pressure created therebetween.
The necessity of saving energy has been recently arousing greater efforts to achieve a reduction in weight of automobile parts. For example, aluminum alloy has come to be used instead of steel for making disk wheels. The demand for e.g., lightweight wheels has grown to the extent. However, it is necessary to consider even a reduction in thickness of aluminum alloy wheels. The mere reduction in thickness of aluminum alloy wheels results also in a lowering of their mechanical strength. It is, therefore, essential to be able to produce aluminum alloy wheels which are small in thickness and yet high in mechanical strength.
The process as proposed in Japanese Patent Application laid open under No. Sho 48-49625, or Hei 3-71964 can produce casting containing a smaller amount of oxide and therefore, having an improved mechanical strength. Their strength is, however, still insufficient for allowing any further reduction in thickness. The process disclosed in Japanese Patent Application laid open under Hei 1-104457 has a drawback of being unable to prevent the oxidation of the molten metal completely, insofar as the surface of the molten metal remains exposed to air until the inert gas is supplied into the stoke. The application contains a statement to the effect that the unsolidified part of the molten metal which has been raised from the holding furnace falls back into the furnace upon elimination of the difference of pressure maintained for raising the molten metal into the mold cavity. The space made open by the falling of the molten metal cannot be vacuum, but necessarily contains air. The purging of the space with the inert gas, which relies upon its specific gravity higher than that of air, requires a certain length of time. And whatever quickly the gas may be introduced, the surface of the molten metal in the holding furnace remains exposed to the air. The oxidation of the molten metal to some extent or other is, therefore, unavoidable. Moreover, the elimination of the difference of pressure does not result in the immediate falling of any and all unsolidified metal, since the gate is sealed by the metal without being completely solidified remaining in the upper portion of the gate. This brings about a delay in the cooling of a cast product near the gate which has an adverse effect not only on the efficiency of production, but also on the formation of a fine metallurgical structure.